What determines eye colour?

Eye colour depends on a single pigment, melanin, found in the iris: there is neither a blue pigment nor a green pigment in the human eye. Light shades result from the scattering of light within the stroma, a purely optical phenomenon comparable to what makes the sky blue. This colour is hereditary but polygenic, it does not follow the simplistic rule of a single recessive gene, and it can change during the first months of life before stabilising.

UNDERSTANDING

What is the role of melanin in the iris?

Eye colour is determined by melanin, the same brown-black pigment that colours the skin and hair. In the iris, this melanin is produced by specialised cells called melanocytes and is deposited mainly in two layers: the stroma, at the front, and the pigmented epithelium, at the back.

What varies from one person to another is not the colour of the pigment, which is always identical, a deep brown-black, but its quantity and its distribution within the stroma. The more melanin the stroma contains, the more the iris absorbs light and appears dark. The less it contains, the more the light scatters and makes the eye appear light.

The pigmented epithelium, the posterior layer of the iris, remains very dark in almost everyone, whatever the visible colour of the eyes. Its role is above all optical: it prevents light from passing through the iris anywhere other than through the pupil, allowing a sharp image on the retina. The colour perceived from the outside is therefore essentially played out in the anterior layer, the stroma.

It is worth noting that the melanocytes of the iris are not variable in number: everyone has roughly the same density of melanocytes. It is the activity of these cells, that is to say the amount of melanin they synthesise and store, that differs. This subtlety explains why a brown iris and a blue iris can share the same cellular structure yet have a radically different visual appearance.

OPTICS

Why are eyes blue, green, grey or brown?

As surprising as it may seem, there is no blue, green or grey pigment in the human eye. All these light shades arise from a physical phenomenon: the scattering of light by the collagen fibres of the stroma, a mechanism comparable to the one that makes the sky blue by scattering sunlight.

Blue eyes and the Tyndall effect

When the stroma contains little melanin, the light entering it is scattered by the collagen fibres. Short wavelengths, in the blue part of the spectrum, are preferentially sent back towards the observer: this is the Tyndall effect, a selective scattering of light by fine particles. The eye appears blue without any blue pigment actually existing in the tissue.

Green and hazel eyes

Green and hazel eyes correspond to intermediate situations. A moderate amount of melanin combines with the scattered light: the yellow-brown of the pigment mixes with the blue of Tyndall scattering, producing a green shade. Hazel, for its part, blends brown and green areas within a single iris, often with variability depending on the ambient light and the size of the pupil.

Grey and brown eyes

Grey eyes resemble blue eyes, but the scattering occurs more uniformly, often linked to a particular density of the collagen fibres of the stroma. Brown eyes, the most common in the world (more than 55% of the world’s population), contain a great deal of melanin: it absorbs most of the light, so that the iris appears brown, from the lightest to almost black.

GENETICS

Is eye colour genetic?

Yes, eye colour is largely hereditary, but it does not depend on a single gene. It is polygenic: several genes work together to regulate the amount of melanin deposited in the stroma of the iris. The textbook model of the “dominant brown gene versus recessive blue gene” is an outdated simplification that explains only a small part of the cases observed.

Two genes located on chromosome 15, OCA2 and HERC2, play a major role in the production and storage of melanin. HERC2 in particular regulates the expression of OCA2: a variation in this gene can be enough to significantly reduce melanin production and give the eyes a light colour. But other genes are involved too, more than a dozen are recognised today, each making a partial contribution.

This complex inheritance has a well-known and often surprising consequence: two brown-eyed parents can have a child with light eyes, and vice versa. A child’s colour is therefore never entirely predictable from that of the parents. Probabilities can be established, statistical tendencies discussed, but the final colour can never be predicted with certainty.

CHANGES

Does eye colour change with age?

Eye colour can change, especially early in life. At birth, the iris of many infants still contains little melanin: this is why so many babies are born with light eyes, sometimes a blue that gradually darkens over the first months.

In infants

During the first months of life, the melanocytes of the iris begin to produce melanin under the effect of exposure to light. The definitive shade usually settles in before the end of the first year, sometimes up to eighteen months. An eye that is blue at birth may thus turn green, hazel or brown as the pigment builds up in the stroma. This transformation is normal and should not be a cause for concern.

In adults

In adults, the colour is in principle stable. The shade may seem to vary with the ambient light, the colour of the clothes worn or the size of the pupil (which changes the proportion of stroma visible), but this is an optical illusion, not a genuine change in pigmentation. On the other hand, a real, one-sided and lasting change of colour in one eye is never trivial and always warrants an ophthalmological examination.

SPECIFIC FEATURES

Heterochromia and colour peculiarities

Some people have atypical or asymmetrical eye colours. Most of these peculiarities are benign and present from birth, but a few reflect a medical cause that it is important to know how to recognise.

• Complete heterochromia: the two eyes are of different colours, for example one blue eye and one brown eye. It is most often congenital, isolated, and without consequence for vision.
• Sectoral heterochromia (or partial): a single iris shows two distinct shades, such as a brown sector on a blue-grey background. It may be congenital or acquired.
• Iris spots and naevi: small, more heavily pigmented areas, comparable to moles, generally stable, benign, and very common in the population.
• Central two-tone iris: a concentric variation in colour, darker at the centre and lighter at the periphery (or the reverse), linked to an uneven distribution of melanin.

Heterochromia that has always been present is rarely worrying. On the other hand, heterochromia that has appeared later in life, or an iris spot that grows, changes shape or alters its relief, warrants an ophthalmological examination without delay. Certain acquired causes, such as inflammation, variations in eye pressure or the effects of some medications, can alter the shade of an iris.

WARNING

When should a change in colour raise the alarm?

A change in the colour of one eye is not trivial when it is accompanied by other signs. Certain symptoms, associated with a change in shade, call for a prompt ophthalmological consultation rather than simple observation.

In these situations, only a slit-lamp examination allows the iris to be observed in detail, the eye pressure to be measured and the cause to be determined. Glaucoma, for example, can in certain forms alter the appearance of the iris and must be detected early. Eye colour, in itself, is not a disease. It is the change associated with symptoms that should prompt a consultation, without delay.

MYTHS

Common myths about eye colour

Eye colour is surrounded by many misconceptions, often rooted in school teaching or passed down from generation to generation. Here are a few that modern genetics and ophthalmology allow us to correct.

“Two blue-eyed parents necessarily have blue-eyed children”

This is inaccurate. Although this scenario is statistically the most common, the polygenic genetics of eye colour leaves a margin. Genetic variants that are carried but not expressed can reappear in a child and give them brown eyes, even if both parents have blue eyes. The phenomenon remains rare, but it is documented.

“Coloured contact lenses are harmless”

Coloured lenses sold without a medical prescription, notably on the internet or in fancy-dress shops, represent a serious risk to the cornea. They are not fitted to the curvature of the eye, they reduce the oxygen supply and they encourage infections. In France, any ocular lens is a medical device subject to an ophthalmological prescription. Corneal topography carried out during an assessment allows, among other things, the fitting compatibility of a lens to be checked.

“Diet can change eye colour”

No scientific data support the idea that a diet, a supplement or a plant can alter the colour of the iris. The pigmentation of the iris is genetically determined and does not respond to nutritional intake. Publications claiming the contrary belong to marketing, not medicine.

FREQUENTLY ASKED QUESTIONS

Frequently asked questions about eye colour

Sources

• French Society of Ophthalmology (SFO) — Anatomy and physiology of the iris, pigmentation and pathologies of the anterior segment.
• College of University Ophthalmologists of France (COUF) — Anatomy of the eye, iris and anterior uveitis.
• Inserm — Genetics of pigmentation, melanin and the determinants of eye colour: role of the OCA2 and HERC2 genes.
• French National Authority for Health (HAS) — Recommendations on ophthalmological care and screening for pathologies of the anterior segment.

This article is for information purposes and does not replace a medical consultation. Only an ophthalmological examination can assess the state of your iris, measure the eye pressure and establish a personalised diagnosis.